The term "molecular sieve" refers to a wide variety of crystalline materials of both natural and synthetic varieties which exhibit the property of acting as sieves on a molecular scale. A major class of molecular sieves are the crystalline aluminosilicates, although other crystalline materials are included in the broad definition. Examples of such other crystalline materials include coal, special active carbons, porous glass, microporous beryllium oxide powders, and layer silicates modified by exchange with organic cations. See, D. W. Breck, "Zeolite Molecular Sieves: Structure, Chemistry, and Use", John Wiley & Sons, 1974.
Zeolites are crystalline, hydrated, framework aluminosilicates which are based on a three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygens.
Zeolites may be represented by the empirical formula EQU M.sub.2/n O.Al.sub.2 O.sub.3.xSiO.sub.2.yH.sub.2 O
wherein, x is generally equal to or greater than 2 since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation, M, and water molecules. The cations may be mobile and exchangeable to varying degrees by other cations. Intracrystalline zeolitic water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and synthetic, cation exchange or dehydration may produce structural changes in the framework. Ammonium and alkylammonium cations may be incorporated in synthetic zeolites, e.g., NH.sub.4, CH.sub.3 NH.sub.3, (CH.sub.3).sub.2 NH.sub.2, (CH.sub.3).sub.3 NH, and (CH.sub.3).sub.4 N. In some synthetic zeolites, aluminum cations may be substituted by gallium ions and silicon ions by germanium or phosphorus ions. The latter necessitates a modification of the structural formula.
The structural formula of a zeolite is best expressed for the crystallographic unit cell as: M.sub.x/n [(AlO.sub.2).sub.x (SiO.sub.2).sub.y ].wH.sub.2 O where M is the cation of valence n, w is the number of water molecules and the ratio y/x usually has values of 1-100 depending upon the structure. The sum (x+y) is the total number of tetrahedra in the unit cell. The complex within the [ ] represents the framework composition.
The zeolites described in the patent literature and published journals are designated by letters or other convenient symbols. Exemplary of these materials are Zeolite A (U.S. Pat. No. 2,882,243), Zeolite X (U.S. Pat. No. 2,882,244), Zeolite Y (U.S. Pat. No. 3,130,007), Zeolite ZSM-5 (U.S. Pat. No. 3,702,886), Zeolite ZSM-11 (U.S. Pat. No. 3,709,979), and Zeolite ZSM-12 (U.S. Pat. No. 3,832,449).
Although there are 34 species of zeolite minerals and about 100 types of synthetic zeolites, only a few have been found to have practical significance. Many of the zeolites, after dehydration, are permeated by very small channel systems which are not interpenetrating and which may contain serious diffusion blocks. In other cases dehydration irreversibly disturbs the framework structure and the positions of metal cations, so that the structure partially collapses and dehydration is not completely reversible. To be efficiently used as a molecular sieve, the structure of the zeolite after complete dehydration must remain intact.
There has been considerable interest in developing metallosilicates other than aluminosilicates which exhibit molecular sieve characteristics. For example, U.S. Pat. Nos. 3,329,480 and 3,329,481 disclose crystalline zircano-silicates and titano-silicates, respectively. U.S. Pat. No. 3,329,384 discloses group IV-B metallosilicates. U.S. Pat. Nos. 4,208,305, 4,238,315 and 4,337,176 disclose iron silicates. U.S. Pat. No. 4,329,328 discloses zinco-, stanno-, and titano-silicates. European patent applications Nos. 0 038 682 and 0 044 740 disclose cobalt silicates. European patent application No. 0 050 525 discloses nickel silicate.
U.K. patent application No. GB 2,024,790 A discloses a silica-based material which has been modified with one or more elements which have entered the crystalline lattice of the silica in place of silicon atoms of the silica or in the form of salts of bisilicic or polysilicic acids. The elements identified as being suitable for making such silica-based materials are chromium, beryllium, titanium, vanadium, manganese, iron, cobalt, zinc, zirconium, rhodium, silver, tin, antimony and boron.
U.S. Pat. No. 4,299,808 discloses chromosilicates formed by reacting an aqueous mixture of an oxide of silicon, a compound of chromium, a hydroxide of an alkali or an alkaline earth metal, and an alkylammonium cation or a precursor of an alkylammonium cation.
There remains a need for a suitable method for preparing metallosilicates that exhibit molecular sieve character whereby the metal is introduced into the crystalline structure in a direct, efficient and primary fashion. Such a method would preferably involve introducing the metal in the original synthesis mixture.